This invention relates to electronic meters and, more particularly, to methods and apparatus for phase compensation in electronic energy meters. The electronic energy meters may be configured for measuring a polyphase power line. Phase errors may be different for different current transformers in the energy meter and may be a function of current in the power line being measured.
Electronic energy meters have been developed for measuring the consumption of electrical energy on power lines. One architecture utilizes a digital signal processor for calculating various electrical parameters based on measured values of voltage and current, and a microcontroller for controlling the storage, display and communication of the electrical parameters calculated by the digital signal processor.
In this architecture, the phase voltages are scaled down to lower voltages using potential dividers, and the line currents are supplied to the primary windings of current transformers. The secondary windings of the current transformers provide current outputs proportional to a turns ratio between the secondary and primary windings of the transformer. A resistor with a very low temperature coefficient connected to the secondary winding of the current transformer provides a potential proportional to the value of the resistance and the current in the secondary winding. These signal conditioned voltage and current signals are supplied to an analog-to-digital converter and are digitized at periodic intervals. The digitized signals are supplied to the digital signal processor to compute instantaneous power and are integrated for a finite number of power line cycles to compute energy.
As inductive elements, the current transformers exhibit a phase lag between the primary and secondary windings. The phase lag may be different for different phases of the power line due to differences between individual current transformers and may be a nonlinear function of line current. Because certain measurements by the electronic energy meter are a function of the phase difference between current and voltage, measurement errors are produced. The errors due to the phase lag therefore adversely affect the accuracy of the electronic energy meter.
Power meter manufacturers typically implement conventional phase shifters using variable resistors and/or variable capacitors for each of the three phases of the power line. This approach increases material and production costs. Techniques have been developed that use two analog-to-digital converters and shift the sampling time of one of the converters with respect to the other. This approach requires additional circuitry and increases the cost of the hardware. These techniques do not compensate for the nonlinearity of the phase shift as a function of current that is exhibited by the current transformers. Prior art phase compensation techniques have been disclosed, for example, in U.S. Pat. No. 5,017,860 issued May 21, 1991 to Germer et al. and U.S. Pat. No. 5,231,347 issued to Jul. 27, 1993 to Voisine et al.
Accordingly, there is a need for improved methods and apparatus for phase compensation in electronic energy meters.
According to a first aspect of the invention, an electronic meter is provided. The electronic meter comprises a sensing circuit for sensing voltage and current values of a waveform, an analog-to-digital converter for converting the sensed voltage and current values to digital voltage and current values, a digital filter for delaying one or both of the digital voltage and current values to compensate for a phase shift error in the sensing circuit, and a computation circuit for computing at least one parameter of the waveform in response to the phase compensated voltage and current values.
The digital filter and the computation circuit may be implemented in a digital signal processor. The sensing circuit may comprise circuitry for sensing voltage and current values of each phase of a polyphase power line. The electronic meter may include a digital filter for delaying the digital voltage values of each phase to compensate for the phase shift error associated with each phase of the polyphase power line. The electronic meter may further comprise means responsive to the current value of the waveform for selecting a set of digital filter coefficients that corresponds to the current value and for applying the selected set of digital filter coefficients to the digital filter. The digital filter may comprise a fractional delay filter for delaying the digital voltage values in increments that are a fraction of the sampling interval of the analog-to-digital converter.
According to another aspect of the invention, a method is provided for calibrating an electronic meter. The method comprises the steps of (a) applying to an input of the electronic meter a test waveform having a known phase shift between voltage and current, (b) measuring a phase shift between the voltage and the current of the test waveform using the electronic meter, (c) determining a phase shift error based on the difference between the known phase shift and the measured phase shift, (d) determining digital filter coefficients to produce a digital filter delay that corresponds to the phase shift error, and (e) saving the digital filter coefficients to compensate for the phase shift error during normal metering operation.
In a preferred embodiment, step (a) comprises applying waveforms of a polyphase power line to the input of the electronic meter and steps (b), (c), (d) and (e) are repeated for each phase of the polyphase power line. Steps (a), (b), (c), (d) and (e) may be repeated for predetermined current ranges to provide a set of digital filter coefficients for each of the predetermined current ranges.
The digital filter coefficients may be determined in a computing device external to the electronic meter and may be downloaded from the external computing device to the electronic meter. In a preferred embodiment, the electronic meter includes a digital signal processor, and the digital filter coefficients are downloaded to the digital signal processor. Step (b) may comprise measuring the power factor of a test waveform with the electronic meter, sending the measured power factor to the external computing device and determining the measured phase shift, in the external computing device, from the measured power factor in the external computing device.
According to a further aspect of the invention, a method is provided for operating an electronic meter. The method comprises the steps of sensing voltage and current values of a waveform using a sensing circuit, converting the sensed voltage and current values to digital voltage and current values, delaying one or both of the digital voltage and current values using a digital filter to compensate for a phase shift error by the sensing circuit, and computing at least one parameter of the waveform in response to the phase compensated voltage and current values.